1. Field of the Invention
The present invention relates to optical communication equipment and, more specifically, to reducing effects of polarization mode dispersion (PMD).
2. Description of the Related Art
Polarization mode dispersion occurs in an optical fiber as a result of small birefringence induced by deviations of the fiber's core from a perfectly cylindrical shape, asymmetric stresses or strains, and/or random external forces acting upon the fiber. PMD is well known to severely impair transmission of optical signals at relatively high bit rates (e.g., 40 Gb/s) over relatively large distances (e.g., 1000 km).
One effect of PMD is that different polarization components of an optical signal at two principle states of polarization (PSP) travel in a fiber at different speeds such that a differential group delay (DGD) is introduced between those components. This effect is generally referred to as first-order PMD. Another effect of PMD is that the shapes of the optical pulses corresponding to different polarization components are distorted differently in the fiber. For example, an optical pulse corresponding to a first PSP may be broadened whereas an optical pulse corresponding to a second PSP may be narrowed. This effect is generally referred to as higher-order PMD. Within higher-order PMD, specific pulse shape distortions corresponding to the second-, third-, etc., orders of PMD may be discriminated. Both first-order PMD and higher-order PMD may significantly distort optical pulses corresponding to optical bits and consequently cause errors at the receiver.
Several techniques have been proposed to date to mitigate the effects of PMD in optical communication systems. Typically, a device known as a PMD compensator is deployed at the receiver end of a fiber transmission link to ensure that the receiver correctly decodes PMD-distorted optical bits.
FIG. 1 illustrates an exemplary prior art PMD compensator 100. Compensator 100 comprises a polarization controller (PC) 102, a DGD element 104, a state of polarization (SOP) monitor 106, and control electronics 108. Depending on the implementation, DGD element 104 may have a variable-length delay line or a constant-length delay line. During operation, PC 102 receives a PMD-distorted optical signal and separates it into two PSP components. DGD element 104 subjects the faster PSP component to a compensating delay to realign it with the slower PSP component. The two PSP components are then recombined and directed to, e.g., a receiver (not shown) for decoding. The output of DGD element 104 is tapped and analyzed by SOP monitor 106, which is configured to provide feedback to PC 102 and possibly to DGD element 104 via control electronics 108. For example, if DGD element 104 has a variable-length delay line, then two feedback signals 110 and 112 are generated by control electronics 108 and applied to DGD element 104 and PC 102, respectively. Based on those signals, PC 102 and DGD element 104 adaptively change their settings to correspond to the dynamically varying amount of PMD in the transmission link. On the other hand, if DGD element 104 has a constant-length delay line, then only signal 112 is generated by control electronics 108 and applied to PC 102. Certain implementations of PMD compensator 100 are described in commonly owned U.S. Pat. No. 5,930,414 by Fishman, et al., the teachings of which are incorporated herein by reference.
A PMD compensator configured with a variable-length delay line generally performs better than one configured with a constant-length delay line. However, it is typically more complex and expensive, and, in certain configurations, may also be less reliable due to the presence of mechanically moving parts in the variable-length delay line. Therefore, what is needed is a practical, cost-effective PMD compensator representing a tradeoff between the performance of variable-length delay line PMD compensators and the simplicity and reliability of constant-length delay line PMD compensators.